I think that Foster has been playing well enough, there's no need to rush him. Give Beachum a little help over on his side and see what happens. Only slide Foster to RT and play DD if absolutely necessary, meaning Beachum is getting destroyed. Which could happen but I think we will be okay. I don't think they will pal DD just to get him some game experience. The best player plays.

Beachum is a rookie making his first start.

DD would be a rookie making his first start.

(Foster plays in either scenario.)

In my mind:
DD & Foster > Beachum & Foster

In other words, DD & Beachum are BOTH rookies... but, DD has far more talent... and, DD was the starter in pre-season (whereas, Beachum is a third-string player). As you said, the best player plays... which would be DD.